Ups and Downs of Dieting
According to a number of studies published in respected journals, your yo-yoing ways have not "ruined" your metabolism. However, all those ups and downs may have changed your body composition in a way that makes it harder for you to lose at each successive cycle. That's because when you drop weight, you lose both fat and muscle, but when you regain weight, you gain back proportionally more fat. That's unfortunate because, pound for pound, muscle burns more calories than fat.
The best way to maintain your valuable muscle mass is to strength train two to three days per week. Also, make sure you're getting enough lean protein. You can achieve this by consuming at least half your body weight in grams of protein. For example, if you weigh 150 pounds, eat no less than 75 grams of protein per day. Best bets are skinless chicken and turkey, seafood, low-fat dairy foods, egg whites, beans, tofu, and edamame.